Nitrogen oxides, such as NO and NO2 (collectively referred to as NOx), generated in the high temperature and high pressure conditions of an internal combustion engine, may constitute a large percentage of total exhaust emissions. In particular, NOx species emitted in the exhaust of lean-burn engines may be particularly difficult to eliminate. Accordingly, engine exhaust systems may utilize selective catalytic reduction (SCR) to reduce the NOx species to diatomic nitrogen and water. A variety of SCR catalysts have been developed including base metal catalysts and zeolite-based catalysts. These catalysts reduce NOx species in the presence of a reducing agent, such as ammonia or urea.
However, the inventors herein have recognized several issues with such catalysts. As one example, during cold start operations and/or at temperatures below catalyst light-off, SCR catalysts may be exposed to a large amount of hydrocarbons (HCs). At these low operating temperatures, zeolite-based SCR catalysts in particular, may adsorb and store a large fraction of the emitted HCs. The stored HCs may degrade the SCR reaction and consequently reduce the NOx conversion efficiency of the catalyst. Furthermore, if not removed from the catalyst, engine operating conditions can trigger an unexpected oxidation of the stored HCs, causing a significant increase in temperature, and possibly permanent thermal deactivation of the SCR catalyst.
In one example, some of the above issues may be addressed by a system for a vehicle including an engine having an exhaust, the system comprising a NOx reducing system coupled to the engine exhaust including a base metal zeolite, said NOx reducing system including a first layer with a first pore size and a second layer with a second pore size, said first pore size being smaller than said second pore size.
As one example, a zeolite formulation configured with a small pore size (e.g., at or less than about 5 Angstroms) may be included as the first layer with the first (smaller) pore size in an engine NOx reducing system. As such, the smaller pore zeolite formulation may or may not include any metals. A base metal zeolite configured with a larger pore size may be included as the second layer with the second (larger) pore size in the engine NOx reducing system. The base metal zeolite may be a transition metal-based zeolite catalyst. As one example, the base metal zeolite may be an Fe-based zeolite catalyst. Alternatively, a Cu-based zeolite catalyst may be employed. The first layer may be positioned between a passage for exhaust gas and the second layer. In this way, the first layer may also constitute an outer layer that may be exposed to exhaust gas before the second layer. The second layer may be positioned between the first layer and a substrate support. In this way, the second layer may also constitute an inner layer that may be exposed to untreated exhaust gas after the first layer. The small pore size of the first layer may impart molecular sieve properties to the layer, thereby buffering and protecting the SCR catalyst in the larger pore sized second layer from large molecular weight hydrocarbons. By reducing the amount of HCs adsorbed, associated exotherms may also be averted. Thus, the use of a first and a second layer of differing pore sizes in an engine NOx reducing system may enable efficient reduction of NOx species by SCR catalysts without the catalysts being adversely affected by exhaust emission HCs. Additionally, in still another embodiment, the first layer may be positioned upstream of the second layer.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.